Production of pig iron

ABSTRACT

A method for producing pig iron by direct processing of iron-containing materials such as iron-containing sands, in which the iron-containing materials and carbonaceous reductant are mixed with a fluxing agent to form a mixture; briquettes or agglomerates are formed from the mixture; at least a portion of the agglomerates are preheated to a temperature of 750 to 1200° C. and are pre-reduced, then the preheated, pre-reduced agglomerates are introduced into the melting furnace; the agglomerates are melted at a temperature of from 1300 to 1760° C. and form hot metal with a slag thereon; the slag is removed and the hot metal is tapped as pig iron, and the off-gas from the smelter is used to operate a preheater for the agglomerates.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of: U.S. Provisional Application No. 61/692,014, filed 22 Aug. 2012; U.S. Provisional Application No. 61/718,510, filed 25 Oct. 2012;

and PCT Application No. PCT/US2013/056078, filed 22 Aug. 2013.

BACKGROUND OF THE INVENTION

A pyro-metallurgical process for treating iron-containing materials, preferably iron sands, iron-containing tailings and upgrades or concentrates thereof, recovers pig iron as well as offers the potential for recovering titanium oxides from the slag. The process requires agglomeration of concentrates of the iron-containing materials with a suitable reductant (e.g., finely ground coal) to form compact agglomerates which are the feed material for an electric smelting furnace.

The agglomerates are melted to form hot metal, principally pig iron, with a slag containing oxides of titanium as well as other mineral species associated with the iron-containing feed material, concentrates, gangue and coal ash. The hot metal is periodically tapped from the electric smelter and cast into solid pig iron to be reclaimed and sold as a merchant product. The slag is also periodically tapped from the smelter, quenched with water, and stockpiled to recover secondary TiO₂ product at a later point in time. The method of TiO₂ recovery from the slag incorporates a low to medium temperature process roast to convert the oxide specie to a compound (typically a chloride salt) that can be dissolved in a solvent (preferably water) and then subsequently precipitated as a pure solid using solvent extraction techniques.

The agglomerates can be either charged ‘cold’ to the electric smelter, or pre-heated in a agglomerate pre-heater and then charged ‘hot’ (up to 1200° C.). Process off-gas from the smelting furnace and the reduction reactor portion of the agglomerate pre-heater can be blended and utilized as a hot combustion fuel to the agglomerate pre-heater. This results in achieving a high level of energy efficiency for the overall process, thereby minimizing the OPEX (operational expenditure) utility cost (primarily electricity purchased from the grid). The spent exhaust gas from the agglomerate pre-heater retains sufficient temperature and sensible heat to act as the drying medium for drying the raw sand for the concentrating plant.

OBJECTS OF THE INVENTION

The principal object of the present invention is to provide an effective method for recovering pig iron from iron-containing feed materials such as iron sands, iron-containing wastes from other metallurgical operations, tailings from mines or concentrators, and concentrates.

Another object of this invention is to recover titanium oxides from slag produced from treatment of such iron-containing materials.

It is also an object of the invention to provide a plant for recovery of pig iron from iron-containing materials which makes maximum utilization of heat created by the process.

Another object of the invention is to provide a means for producing all the required electricity to accommodate the process and operate the plant, including the electric smelter, in such manner as to be electricity self-sufficient.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects will become more readily apparent by referring to the following detailed description and the appended drawings, in which:

FIG. 1 is a schematic flowsheet of the process for recovering pig iron from the process of the invention.

FIG. 2 is a chart showing a data plot of the reduction kinetics for the iron sands at one location.

FIG. 3 is a schematic flowsheet of an alternative process for recovering pig iron.

DETAILED DESCRIPTION

The reference numerals in FIGS. 1 and 3 refer to the following items:

-   10—iron-containing materials, such as iron sands, wastes, tailings,     upgrades or concentrates, -   12—prepared reductant, such as coal, coke, petroleum coke, char,     etc., -   14—fluxing agents—such as CaO, MgO, CaF₂, Al₂O₃, SiO₂, etc. -   18—binder such as cellulose, bentonite, molasses, starch—either     organic or inorganic -   20—mixer -   21—mixture -   22—briquetter or agglomerater -   23—cold briquettes to smelter -   24—conveyor to smelter conduit -   26—conveyor to pre-heater conduit -   27—cold briquettes to pre-heater -   28—preheater -   30—preheated briquettes -   32—electric smelter -   34—hot metal -   36—slag -   40—hot off-gas from smelter -   42—smelter off gas cooler-scrubber -   43—combustible gas -   44—natural gas -   46—flue gas from preheater -   48—heat exchanger -   50—source of combustion air -   54—waste heat boiler -   56—collected hot off-gas -   58—stack -   60—raw material dryer -   62—dry raw material -   64—incinerator -   66—combustion air

The process has the following steps:

1. Feed preparation and agglomeration of sized iron sand concentrate with sized carbon reductant and sized flux agents (if necessary) using an appropriate binder. The preferred agglomeration method is cold briquetting.

2. Feed briquetted agglomerates to a moving hearth agglomerate pre-heater, such as a rotary hearth furnace or a straight tunnel furnace that is fired by combustible fuel gas produced entirely or principally from the electric smelting operation. Pre-heating of the charge, as well as pre-reduction of the iron oxide contained within the agglomerates, results in a decrease of the smelter specific electrical consumption.

3. Feeding pre-heated agglomerates to the electric smelter.

4. Heating the smelter charge to produce carburized liquid iron, liquid slag and combustible off-gases.

5. Tapping liquid carbon containing pig iron from the smelter vessel on a periodic or intermittent tap schedule.

6. Periodic tapping of liquid slag from the electric smelter for granulation and downstream processing, or disposal. The expected concentration of TiO₂ in the slag suggests economic viability for downstream recovery, but this is not a prerequisite for or essential to the process flow sheet.

7. Utilizing blended smelter and agglomerate pre-heater reduction off-gas as a low pressure combustible fuel for the indirect fired agglomerate pre-heater.

In summary, the process is basically a pyro-metallurgical treatment of the iron-containing concentrate which eliminates titanium and vanadium normally associated with concentrate material and promotes the production of high purity liquid hot metal or merchant pig iron that can be utilized in downstream steelmaking operations. The process has the features of either utilizing the electric smelter off-gas for preheating the smelter charge, or generating electricity by combusting the high calorific value off-gas from the smelter using known gas turbine technology. Therefore, the process can produce much (and possibly all) of the electricity required by the plant. Thus the technology should qualify for carbon credits as well.

Referring now to FIG. 1, iron-containing materials 10, along with prepared reductant 12, such as coal, thermal coal, low rank coal, lignite, peat, coke, petroleum coke, or char, fluxing agents 14, such as CaO, MgO, CaF₂, Al₂O₃, SiO₂, etc., and optionally a binder 18, such as cellulose, bentonite, molasses, or starch - either organic or inorganic, are fed to a mixer 20 to form a mixture 21.

Advantageously, the iron-containing materials 10 are screened to pass 80-mesh Tyler Standard. Preferably, 100% of the iron-containing materials pass 10-mesh Tyler Standard (1.70 mm); 100% of the carbonaceous reductant is minus 25 mm; and 100% of the fluxing agent is minus 25 mm.

The mixture 21 is introduced to agglomerater or briquetter 22 in which cold briquettes 23 or agglomerated pellets are formed by agglomeration or cold pressing. The briquettes 23 can be fed cold into electric smelter 32, or they can be preheated in a preheater 28, then fed as hot briquettes 30 into the smelter. Either cold or preheated briquettes or agglomerates, or both cold and preheated briquettes, can be fed to the smelter, which refines the iron-containing materials. Slag 36 is drawn off from the smelter, and pig iron 34 is tapped periodically on an intermittent basis, as is removal of the slag.

The preheater 28 can be a rotary hearth furnace, or alternatively can be a tunnel furnace through which moving grates pass, or which may incorporate trays on a straight grate or other means for conveying the briquettes through the preheater. Sand seals can be provided for the tunnel furnace to maintain and preserve the proper atmosphere. The preheater operates at a temperature range of about 750-1200° C., and the briquette residence time is 15 to 40 minutes. The preheater actually accomplishes pre-reduction of the iron values in the briquettes, with metallization ranging from about 35% to about 90% depending on the operating temperature and the residence time. When the preheater operating temperature is 1000° C., the metallization of the iron values is about 70 to 80% iron (Fe). When the reductant 12 is high-rank coal, a higher processing temperature is required. Using thermal coal in the briquettes allows a shorter residence time in the preheater, and it can operate at lower temperatures with good metallization of at least 70%. When the feed to the preheater is comprised of homogeneous pellets, a deep bed of such pellets can be formed, while still achieving an average metallization of about 70%.

Referring now to the data plot of FIG. 2, excellent metallization (−90%) has been achieved at 1100° C. after a residence time of 30 minutes and a respectable 81% metallization after just 15 minutes residence time. This is excellent kinetics at relatively low temperatures. The process works well even at about 50% metallization. The lower the temperature of the pre-heat/pre-reduction step, the less stress on the equipment. Operating at these temperatures with a rotary hearth furnace preheater requires no chill plate in the rotary hearth furnace. This allows the process to use a small rotary hearth, which has a very positive impact on CAPEX (Capital Expenditure) costs.

The hot briquettes are discharged from the pre-heat furnace at a high temperature, preferably about 1,100 to 1,200° C., and then conveyed to a storage/buffer hopper 30 and then finally metered into the electric smelter by a feeding system (lock hoppers/ wiper bar/ etc.). Hot off-gas 40, which contains combustibles CO and H₂, is removed from smelter 32 at a temperature ranging from 1,000 to 1,600° C. The combustible-containing gas is cleaned, modified and/or tempered to a temperature of about 1,000-1,200° C. in cooler-scrubber 42, then used as the heating gas in preheater 28. Natural gas from source N may be added to the hot fuel gas 40, if necessary, or as desired. Flue gas 46 from preheater 28 is utilized in a heat exchanger 48 to preheat additional combustion air 50 for the preheater 28, and also in a waste heat boiler 54 for the production of high and low pressure steam. Off-gas from both the heat exchanger 48 and the waste heat boiler 54 is collected at 56. Unwanted hot off-gas can be delivered to stack 58, but preferably the collected off-gas is conducted to and used in raw material dryer 60 to dry the raw iron-containing feed material 10 before delivering the dried feed material to the mixer 20.

Off-gas exiting the waste heat boiler 54 and the heat exchanger 48 may be compressed in high pressure compressor and used as combustion fuel in a gas turbine which drives a generator to produce electricity. Sensible heat contained in any hot off-gas in the process may be recovered in a waste heat recovery boiler system. The waste heat boiler system steam cycle could be a “Kalina” cycle based on using 70% ammonia and 30% water for better range processing and heat recovery efficiency at lower gas temperatures.

Ammonia/water boiling occurs over a range of temperatures rather than at a specific temperature and pressure. Steam produced by the waste heat boiler system is then used to drive a steam turbine and generator to produce additional electricity. One of the objectives realized by the invention is to produce most of the required electricity to accommodate the process and to operate the plant so as to be nearly electricity self-sufficient.

Waste off-gas may be collected from each location in the process where it is emitted, and delivered to a stack such as stack 58 in which the off-gas is combusted to convert carbon monoxide and hydrogen to carbon dioxide and water vapor.

SUMMARY OF THE ACHIEVEMENT OF THE OBJECTS OF THE INVENTION

From the foregoing, it is readily apparent that I have invented an improved process for recovering pig iron from iron-containing sands more effectively than heretofore, as well as a plant and apparatus for recovery of pig iron from iron-containing sands which makes maximum utilization of heat created by the process. 

What is claimed is:
 1. A process for producing pig iron by direct processing of iron-containing materials, comprising the steps of: a. mixing iron-containing materials, carbonaceous reductant, and a fluxing agent to form a mixture; b. forming agglomerates from said mixture; c. introducing a portion of said agglomerates to an electric melting furnace as cold charge; d. preheating and pre-reducing at least a portion of said agglomerates to a temperature of 750 to 1200° C., and introducing said preheated agglomerated to the melting furnace; e. melting the agglomerates at a temperature of from 1300 to 1760° C. and forming hot metal with a slag thereon; f. removing the slag; g. tapping the hot metal as pig iron, and h. recovering the heating value from the off-gas from the smelter.
 2. A process according to claim 1, further comprising screening the iron-containing materials to pass 80 mesh Tyler Standard.
 3. A process according to claim 1 wherein said iron-bearing materials are iron sands.
 4. A process according to claim 1, further comprising introducing a binder into said mixture.
 5. A process according to claim 1, further comprising preventing substantially all air ingress to the melting furnace by providing a pressure seal.
 6. A process according to claim 1, further comprising forming hot off-gases in the melting furnace, cleaning and cooling the off-gases, and utilizing the cleaned off-gases as the fuel gas to preheat the preheated portion of the agglomerates.
 7. A process according to claim 3, wherein: 100% of the iron-containing sands pass 10 mesh Tyler Standard (1.70 mm); 100% of the carbonaceous reductant is minus 25 mm; and 100% of the fluxing agent is minus 25 mm.
 8. A process according to claim 1 wherein the carbonaceous reductant is selected from the group consisting of coal, coke, petroleum coke, and char.
 9. A process according to claim 1, wherein the fluxing agent is selected from the group consisting of CaO, MgO, CaF₂, Al₂O₃, SiO₂, and mixtures thereof.
 10. A process according to claim 4, wherein the binder is selected from the group consisting of cellulose, bentonite, molasses, starch or mixtures thereof.
 11. A process according to claim 1, further comprising maintaining a reducing atmosphere within said melting furnace.
 12. A process according to claim 1, further comprising recovering hot off gases from the preheater and passing them through a waste heat boiler to form steam, and utilizing the steam to drive a steam turbine and/or a generator to produce electricity.
 13. A process according to claim 1, wherein the preheater is a rotary hearth furnace.
 14. A process according to claim 1 wherein the preheater is a tunnel furnace.
 15. A process according to claim 1, wherein the iron-containing sands contains titanium values which report to the slag, further comprising recovering the titanium values from the slag.
 16. A method for producing pig iron by direct processing of iron-containing sands, comprising the steps of: a. mixing iron-containing sands, carbonaceous reductant, and a fluxing agent to form a mixture; b. forming agglomerates from said mixture; c. preheating and pre-reducing at least a portion of said agglomerates to a temperature of 750 to 1200° C., and introducing said preheated agglomerated to a smelting furnace; d. melting the agglomerates at a temperature of from 1300 to 1760° C. and forming hot metal with a slag thereon; e. removing the slag; and f. tapping the hot metal as pig iron.
 17. A method according to claim 16, further comprising forming an off-gas in the smelting furnace, removing the off-gas and recovering the heating value from the off-gas from the smelting furnace.
 18. A method according to claim 16, further comprising screening the sands to pass 80 mesh Tyler Standard.
 19. A method according to claim 16, further comprising introducing a binder into said mixture.
 20. A method according to claim 16, further comprising preventing substantially all air ingress to the melting furnace by providing a pressure seal.
 21. A method according to claim 16, further comprising forming and removing off-gases in the melting furnace, cleaning and cooling the removed off-gases, and utilizing the cleaned off-gases to preheat the preheated portion of the briquette charge.
 22. A method according to claim 16, further comprising introducing a portion of said agglomerates to the melting furnace as cold charge.
 23. A method according to claim 16 wherein fuel for preheating said agglomerates is selected from the group consisting of natural gas, cleaned and cooled off-gas from the melting furnace, or a combination thereof.
 24. A method according to claim 16, wherein: 100% of the iron-containing sands pass 10 mesh Tyler Standard (1.70 mm); 100% of the carbonaceous reductant is minus 25 mm; and 100% of the fluxing agent is minus 25 mm.
 25. A method according to claim 16 wherein the carbonaceous reductant is selected from the group consisting of coal, coke, petroleum coke, and char.
 26. A method according to claim 16, wherein the fluxing agent is selected from the group consisting of CaO, MgO, CaF₂, SiO₂, Al₂O₃, and mixtures thereof.
 27. A method according to claim 19, wherein the binder is selected from the group consisting of cellulose, bentonite, molasses, starch or mixtures thereof.
 28. A method according to claim 16, further comprising maintaining a reducing atmosphere within said melting furnace.
 29. A method according to claim 16, further comprising recovering hot off gases from the preheater and passing them through a waste heat boiler to form steam, and utilizing the steam to drive a steam turbine and/or a generator to produce electricity.
 30. A method according to claim 16, wherein the preheater is a rotary hearth furnace.
 31. A method according to claim 16 wherein the preheater is a tunnel furnace.
 32. A method according to claim 16, wherein the iron-containing sands contains titanium values which report to the slag, further comprising recovering the titanium values from the slag.
 33. A method according to claim 21, wherein the removed off-gases from the melting furnace are cooled to a temperature of from about 25° C. to about 1,200° C.
 34. A method according to claim 16, further comprising recovering TiO₂ from the slag by converting the titanium values to a salt by a medium temperature roast, dissolving the oxide in a solvent, then precipitating TiO₂ as a solid by solvent extraction. 